Truck trailers used to transport perishable goods include a refrigerated trailer pulled behind a track cab unit. The refrigerated trailer, which houses the perishable cargo, requires a refrigeration unit for maintaining a desired temperature environment within the interior volume of the container. The refrigeration unit must have sufficient refrigeration capacity to maintain the product stored within the trailer at the desired temperature over a wide range of ambient air temperatures and load conditions. Refrigerated trailers of this type are used to transport a wide variety of products, ranging for example from freshly picked produce to deep frozen seafood. Product may be loaded into the trailer unit directly from the field, such as freshly picked fruits and vegetables, or from a warehouse.
Conventional transport refrigeration systems used in connection with truck trailers include a refrigeration unit operatively associated with the trailer. The refrigeration unit includes a refrigerant compressor, a condenser coil, an expansion device, commonly a thermostatic expansion valve (TXV), and an evaporator coil connected via appropriate refrigerant lines in a closed refrigerant flow circuit. The refrigeration unit is generally contained in a framework that is attached to the front wall of the trailer behind the truck cab such that the air or gas/air mixture or other gas within the interior volume of the trailer may be circulated over the evaporator coil by means of an evaporator fan associated with the evaporator coil which is disposed within the interior of the trailer, typically mounted in an opening in the front wall to which the refrigeration unit is attached. The transport refrigeration system also includes an electric generator driven by a diesel powered engine and adapted to produce AC current at a selected voltage and frequency to operate a compressor drive motor driving the refrigeration compressor. For example, U.S. Pat. No. 6,223,546 discloses a transport refrigeration unit configured to be mounted on the front wall of a refrigerated transport trailer.
Generally, products loaded into a trailer from a warehouse have already been cooled to the desired product storage temperature within a refrigeration facility at the warehouse. When the chilled products have been transferred into the trailer, the trailer doors are closed and the refrigeration unit is operated in a “pulldown” mode to rapidly reduce the air temperature within the trailer from ambient or near ambient temperature down to the desired product storage temperature for transport. In conventional practice, the refrigeration unit is typically designed with a refrigeration capacity sized to provide stable temperature at a low box temperature desired for a frozen product and even a deep frozen product, which would be at least as low as 0° C. (32° F.) and as low as −18° C. (about 0° F.) for a deep frozen product.
However, when product which has not been pre-cooled, such as for example fresh fruits and vegetables direct from the field, is loaded into the trailer “hot”, that is at an ambient temperature above the desired product storage temperature, such as for example directly from the field at ambient outdoor temperature, a substantial refrigeration load is imposed upon the transport refrigeration unit operating in the pulldown mode to not only reduce the temperature of the air within the trailer interior, but also to reduce the temperature of the product down from ambient outdoor temperature to the desired product storage temperature. Products loaded “hot” require the refrigeration unit to pull the product temperature down to set point temperature, typically about 2° C. (about 36° F.) from product temperatures ranging as high as 38° C. (100° F.) or above.
During pull down, the refrigerant unit requires high refrigerant mass flow to cool the trailer interior and the product down as rapidly as possible. Thus, the refrigerant compressor must operate at or near its maximum load capacity. With the refrigerant compressor operating at or near its load capacity, particularly when the ambient outdoor temperature is high, the risk exists that the electric current drawn by the refrigerant compressor may exceed the maximum current limit.